COVID-19: Data suggesting Omicron causes milder illness than Delta is good news but two big questions remain

·3-min read

At this stage of the Omicron wave, it's as good a Christmas gift as we could have hoped for.

The first studies of real-world hospitalisation data from Scotland and England suggest the Omicron variant does cause milder illness than the Delta variant it is fast outpacing.

But - and it's a huge but - we still don't know whether this means we'll still see post-Christmas restrictions or an NHS breaking under the strain.

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First the good news. Analysis from the EAVE study based at the University of Edinburgh, which covers the medical histories of 99% of the population of Scotland, finds that overall, the risk of hospitalisation due to Omicron may be a whopping 56% lower compared to Delta.

The confidence in that estimate is quite low. The margin of error ranges from 30% to 75% reduction in risk. That's simply because fewer than 20 people have been admitted to hospital with Omicron in the whole of Scotland.

Significantly, hardly anyone over the age of 60 has been hospitalised so far either, so it's hard to know if that apparent reduction in risk will get lower as older people get infected - or higher given most people over 60 in Scotland have been boosted, which improves protection.

What was hard for the Scottish analysis to do was assess whether Omicron was intrinsically more mild than Delta, or just looks that way because so many people in Scotland are either vaccinated or had been previously infected.

That's where another hot-off-the-press analysis from Imperial College in London comes in. Due to huge numbers of Omicron cases in London and rapidly rising rates of hospitalisation in the capital, they had more raw data to draw on.

Their analysis was a bit more nuanced. It found the risk of an overnight stay in hospital was about 40-45% lower with Omicron than Delta. And a reduction in the risk of a day visit by around 15 to 25%.

But the Imperial researchers were able to estimate that if you had no immunity at all (either through vaccination or prior infection) you were around 11% less likely to go to hospital with Omicron than with Delta.

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So, good news, right?

"It's qualified good news," says Dr Jim McMenamin of Public Health Scotland.

Two big questions still remain. First, will the apparent reduction in severity hold up as more people over the age of 60 get infected with Omicron?

Comparatively few people of that age group are included in either analysis, making their conclusions statistically quite weak.

But, with other variants, the over-60s are much more likely to get severe illness or die.

The spread of Omicron began in younger adults. It's possible, but unlikely, that it will stay there, because the older group are so well vaccinated in the UK.

But if more over-60s do end up getting infected, the data could start to show Omicron is more severe in that age group.

The other big question is how large the Omicron wave becomes.

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Case rates are still rocketing in most parts of the UK. Even if Omicron is 50% "milder" than Delta, if it infects three times more people, you're still going to get more going into hospital overall.

Even if their disease is milder and their stay shorter, if the number of cases is high enough, it could still be a problem for the NHS.

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